Geometric limitations

In addition to the differences dictated by the different metallurgical processes, the presence of the refractory mold in the casting process with the entire chain of operations necessary for its manufacture and filling by the metal is a source of geometric limitations of the jewel, which can be overcome with selective laser casting. An example is the impossibility to produce a unique piece jewelry with a hollow interior almost insulated from the outside.

a. Classic and Direct Microfusion

One of the biggest limitations of classical microfusion is the impossibility to create hollow objects with minimal thicknesses. Even in direct casting the problem of the production of hollow items persists, despite the fact that resins can be built with cavities almost insulated from the outside - because they are obtained with an additive printing technique - it is necessary to equip the model with numerous holes. These are essential in the next phase of cylinder production the coating where the liquid is not able to penetrate into the internal cavity except through the holes.

A typical practical solution to circumvent the limitations imposed by casting hollow jewelry from combustible material models (wax, resin) is the separate casting of the partitions of the object, which will then be soldered. However, soldering involves the appearance of additional problems, in addition to the presence of an additional production step. The problems generated in this case are related to the need to mask the joint area, often of a color not corresponding to the adjacent material. In addition, the heating caused by the welding operation can lead to deformation and breakage of the parts. Finally, there is a real risk that in the case of thin thicknesses, welding will cause the accidental fusion of a large area of the jewel.

b. Selective Laser Melting

In SLM the problems of microcasting decay and in the construction of hollow items the only need is to drill a limited number of tiny holes to allow the escape of the dust trapped inside the object obtaining without particular production problems almost totally closed hollow forms, such as the rings in Figure 5.1 and the ring in Figure 5.2, which shows the internal cavity that can be filled or not with reticular support structures to increase the mechanical strength. The ability to produce items with occluded cavities is a recognized strength in laser printing because it offers the possibility to reduce the final weight of the jewel with the same overall size.

Figure 5.1 – Example of hollow faith made by SLM with a small hole (3D model on the left and printed object on the right)

Source: see [28]

Figure 5.2 – White gold ring with hollow inside, cross- section with and without reticular support structure.

Source: see [28]

5.3.2 For thicknesses

The presence of the phases of making the model in wax or resin, the production of ceramic coatings and the casting of the molten alloy inside them also leads to a further limitation in the design of the jewel, which are the thicknesses.

a. Classic and Direct Microfusion

The production of jewelry with thin thicknesses using the classic method makes the production of the model problematic because its filling requires conditions of high wax compression, so the rubber tends to swell and produce thicker or deformed models. In order to overcome this problem, rubber moulds have been designed to facilitate filling, which combined with the adoption of waxes with filling properties make the process more feasible [32]. However, the presence of thin thicknesses always leads to a considerable difficulty in extracting the model from the molds, with frequent and irreparable distortions and dimensional changes.

In the case of direct casting the adoption of additive manufacturing makes the production phase of the model much less critical, with a correct choice of polymeric materials and printing technique, high quality castings are obtained without resin residues that can obstruct the thin cavities then to be filled. [33]

b. Selective Laser Melting

In the case of laser selective casting, thin thicknesses and complex structures are not a limiting factor, as a wax model and refractory mold filling are not required. The limit of the metal walls is given by the thickness of the single trace of laser melting, compatible with the preservation of the adequate strength required by a jewel and the type of metal powder.

SLM not only revolutionizes the way to design and conceive shapes, but also to optimize the investment of the precious metal present in the jewel.

In microfusion it is therefore necessary to renounce a priori certain jewelry geometries because the risk of failure and therefore waste is very high: the number of incomplete pieces can increase drastically within a casting containing many dozens of pieces and the success in the production of a single jewel cannot be translated into the success of mass production.

With the new laser technology, on the other hand, there is no dimensional constraint other than that dictated by the precision of the laser, a problem that is destined to be extinguished with the perfection of the technology.

5.3.3 For supports

In both classic/direct casting and SLM, the presence of supports and power supplies necessary for the success of the jewel cannot be ignored. These elements are indispensable to fix the growth position of the jewelry and send the metal alloy into the hollow shapes mounted on the shaft.

However, it is important to note that particular jewelry geometries may require such a support that the relationship between the volume of the pieces and the volume of the supports is disadvantageous.

Like casting feeders, support scaffolds are a valuable part that must be removed from the jewelry and therefore their quantity is a factor that must be considered in the overall economy of the production process as an integral part of production waste.

Figure 5.3 – Sample of a ring with structure support and injection feeders

Source: see [5]

a. Classic and Direct Microfusion

Unlike classic casting, the media is generally also required in direct casting for printing the model in wax or resin. In this case the supports are removed before the production of the plaster molds and are therefore not present in the assembled shaft, thus influencing the production process only

Injection feeders Structural

support

in terms of the time needed for their removal and not on the quality of the workpiece and of the processing waste.

b. Selective Laser Melting

In laser printing, the size and number of media is only related to the surface area of the objects and is necessary to support the cantilevered parts of the object, which otherwise could be easily knocked out by the movement of the metal dust distribution brush. The shape of the supports and their position is designed to make it easy and economical to remove the parts from the construction platform. Their shape mainly takes the appearance of three-dimensional grids, with very thin contact points on the workpiece.

In this case, production waste in the form of supports does not increase as the mass of the object increases, since it is related to the overall surface of the workpiece instead of its volume. The main disadvantage related to the presence of supports in selective casting is due to their removal phase. A residue of their presence may in fact remain attached to the printed jewel or may remain tiny craters. The extent of these defects can be eliminated during the finishing phase (Figure 5.4).

Figure 5.4 – Surface of the bottom surface of rings - printed with different types of support - after manual removal of the media. Holes left by media removal are visible in zones 1 and 2

Source: see [28]

Further drawbacks occur when the support is too far away or too thin, landslides,

displacements and overheating of the piece may occur with compromise of the final quality

of the jewel, if instead the support is too much the roughness of the affected surfaces may be high and its removal difficult.